1. FIELD OF THE INVENTION
The field of this invention relates to logging instruments for ascertaining physical properties of media such as the earth surrounding an oil well borehole, and more particularly to a logging instrument which utilizes non-nuclear radiation to determine the physical properties of the media.
2. DESCRIPTION OF THE PRIOR ART
The measurement of material bulk density, lithology and other material properties as a function of depth in boreholes within the earth is of major importance for analyzing oil and gas reservoirs, aquifers and mineral deposits. Radioactive photon sources, which release gamma-rays, are currently used extensively in the measurement of formation properties such as bulk density and lithology in boreholes. These properties are determined by measuring the intensity of the gamma-rays that are scattered from the formation surrounding the borehole. Radioactive logging tools typically use one to two curies of 137Cs or 60Co as the source of gamma-rays. Stronger sources, containing more radioactive material, are usually precluded due to the safety hazards associated with handling and transporting of such radioactive tools.
It is common for boreholes to be between ten thousand and thirty thousand feet in depth. Sometimes during the operation of these tools, the tool becomes lodged in the borehole and the tool can not be easily removed. Even though these tools are reasonably expensive, it may be economical to merely close the borehole and lose the tool. However, environmentalists express substantial dissatisfaction with the locating of a radioactive source deep within the earth. It is a concern that this radioactive source, because of its inherently long radiation emission lifespan, will contaminate underground water. Therefore, there has been substantial interest in developing a non-nuclear type of logging tool that, in case the tool becomes lost deep within the earth, can be merely abandoned with such abandonment being safe (not creating any kind of a hazard).
In addition, it would be desireable to have tools with higher source intensity in order to facilitate higher logging speeds. Higher intensities are not practical with radioactive tools due to the increased hazards.
Previously, work has been performed to develop a small, high energy electron beam source to create an intense x-ray stream which could be used in logging tools (Turcotte, U.S. Pat. No. 3,976,879; Turcotte, U.S. Pat. No. 4,093,854). This work utilizes scattering of x-rays in the media of the borehole in a manner similar to the way that the gamma-rays have been used to determine the various properties of the media. These prior art approaches utilized a high energy electron beam which is directed onto an x-ray target where the x-rays are produced.
Creation of the x-rays is accomplished using the high energy electron beam. There are several types of electron accelerators which can be considered for use in such a tool. The simplest is the diode accelerator in which electrons are accelerated to high energies in a single gap between an electrode which is biased at a negative high voltage relative to a grounded electrode. Electrons are drawn from the negative electrode and accelerated across the inter-electrode gap and, by providing a hole in the grounded electrode, electrons can be extracted to form a beam. One of the disadvantages of diode accelerators is that such require a voltage comparable to the beam voltage, which therefore, requires a high voltage source. The usage of high voltage and a high voltage source encourages electrical breakdown in the tool. Typically the diode accelerator requires approximately one megavolt where the tool of the present invention requires only fifty kilovolts. Additionally, a diode accelerator is difficult to produce with a compact structure. Since one of the primary usages of the present invention is within the limited space of an oil well borehole, it is difficult to construct a diode accelerator to be used within such a limited space environment.
Recirculating accelerators, including betatrons and synchrotrons, can be considered for use to generate x-rays within a logging tool. Betatrons and synchrotrons accelerate electrons as such pass repeatedly around a closed accelerating path. Again, size limitation is the main drawback to such accelerators.
Linear accelerators appear to be the most feasible in conjunction with non-nuclear logging tools. Linear accelerators are limited to radio frequency and induction types. Substantial work has been completed in the past with radio frequency accelerators for application to logging operations. Reference is to be had to the aforementioned Turcotte patent (U.S. Pat. No. 4,093,854).
Radio frequency accelerators contain electrically resonant structures which must be tuned accurately to the frequency of the radio frequency generator which is used to energize the accelerator. The radio frequency generator can be operated either continuously or in a pulsed mode. Because of the sensitivity of radio frequency linear accelerators to tuning, sophisticated mechanisms must be provided to compensate for changes in tuning as the dimensions of the accelerating structure changes due to changes in temperature. Logging tools must be capable of operating over a temperature range of between 0.degree. Centigrade and 175.degree. Centigrade. Another patent which shows a similar type of radio frequency accelerator in conjunction with the logging tool is Haimson, No. 4,713,581.
One of the primary disadvantages to radio frequency linear accelerators in logging tools is their expense. Inherently such are quite complex and, therefore, quite expensive. The estimate at the present time is for such tools to cost as much as $250,000.